Network nodes of a data network are generally provided with a clock that is used to provide timing information to the network node, for example to provide timing information for the generation of outgoing data units by the network node.
In a synchronous data network, such as a Synchronous Optical Networking (SONET) network or a Synchronous Digital Hierarchy (SDH) network, the clocks of the nodes of the same network are synchronised. This may be achieved by the network nodes extracting clock timing information from signals received from other network nodes and using the extracted clock timing information to synchronise the clock of the network node to the received clock timing. Information about the quality of the clock used to generate the received signals may be included in the signals to enable a network node to select the highest quality received clock with which to synchronise.
In an asynchronous network, the network nodes typically have a local clock which is used when generating signals for onward transmission. However the local clocks of the network nodes are not synchronised with each other and therefore there is no need to extract clock quality information from received signals.
In some situations two synchronous networks may be connected together only by asynchronous networks. FIG. 1 shows a networks arrangement 2 in which a first synchronous network 4 and a second synchronous network 6 are connected via an asynchronous network 8. Network nodes 10 and 12 are edge nodes of the asynchronous network 8 and data sent from a network node 14 of the synchronous network 4 to the network node 16 of synchronous network 6 is routed via the network nodes 10 and 12.
Such a situation may arise, for example in the case of two synchronous SONET/SDH networks that are connected only by an asynchronous Optical Transport Network (OTN), for example using ITU-T G.709: Interfaces for the Optical Transport Network (OTN) standard.
It is desirable to be able to transfer both clock timing information and clock quality information between synchronous network nodes such as network node 14 and network node 16 via an asynchronous network 8.
The ITU-T G.709: Interfaces for the Optical Transport Network (OTN) standard has recently been enhanced to assure the timing transparency of the transported signals. As a result it is now possible to map signals from a synchronous network such as a SONET/SDH network 4 synchronously into a data unit for transport within a network 8 conforming to the ITU-T G.709: Interfaces for the Optical Transport Network (OTN) standard, for example at an edge node 10 shown in FIG. 1. The data unit may then be routed through the asynchronous network conforming to the ITU-T G.709: Interfaces for the Optical Transport Network (OTN) standard to edge node 12 as the entry point into a second SONET/SDH network 6. When the data unit reaches the edge node 12, the SONET/SDH network signal may be unmapped from the data unit of the network 8 conforming to the ITU-T G.709: Interfaces for the Optical Transport Network (OTN) standard in a synchronous manner so that the clock information is thus transported via the asynchronous OTN network 8 between the two synchronous networks 4, 6. However, there is currently no mechanism for transporting clock quality information over a network conforming to the ITU-T G.709: Interfaces for the Optical Transport Network (OTN) standard such as network 8 shown in FIG. 1.
In some data networks, a number of lower order data units, each lower order data unit having an associated clock, may be combined to form a higher order data unit. However, there is currently no mechanism for transporting clock quality information over a network in which a number of clocks may be associated with a data unit. An example of a network in which a number of lower order data units are multiplexed into a higher order data unit is a network conforming to the ITU-T G.709: Interfaces for the Optical Transport Network (OTN) standard.
It may be desirable for a network node to obtain clock quality information for clocks associated with data units received at the network node, in order to synchronise the network node local clock to a clock associated with a received data unit.
The present invention seeks to alleviate at least some of the problems of the prior art and to provide a method and apparatus for forming and processing data units having clock quality information.